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Re: Measuring S11 or frequency range of an active SRL
Jinze,
My understanding is you used mini-circuits RF transformer that has winding ratio 1:1 to connect VNA to your designed LZ1AQ amplifier with balanced input and measured Z input of your antenna amplifier.? Your plot shows a peak of measured impedance |Z| = 244.5 ohm at frequency 22 MHz. I do not follow? your explanation "this is more in line with the impedance matching expactation of the loop". How come ? Impedance matching for a broad band application can be discussed? when Ra + j Xa of? antenna impedance can transformed in R inp + j X inp in such way to achieve a maximum power transfer when?4 R a x R inp / | Z a + Z inp | ^ 2 = 1 - |Gamma|^2 for a specified bandwidth from f min to f max of design interest. Have you ascertained that ? That is the function of matching network between antenna and input of your amplifier. Raphael |
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Re: Measuring S11 or frequency range of an active SRL
?
¸é²¹±è³ó²¹±ð±ô£¬ I actually use TC1-1T+ as the input coupling of the loop amplifier to test the input impedance characteristics of the LZ1AQ amplifier I made I think this is more in line with the impedance matching expectations of the loop.
This is relatively simple compared to constructing an environment without electromagnetic interference to test air characteristics |
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Re: Measuring S11 or frequency range of an active SRL
Yep.? While working for a living (I'm retired) the companies we worked for paid for the good, high end, and calibrated antennas, loops (for LF), and peripheral equipment.? I presently have a small set but no LPDA covering below 300 MHz.? We can always fall back on the substitution method with a bit more work. Dave - W?LEV On Mon, Dec 5, 2022 at 10:00 PM Raphael Wasserman <wassermanr46@...> wrote:
-- Dave - W?LEV |
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Re: Measuring S11 or frequency range of an active SRL
¿ªÔÆÌåÓýDave, ? The pleasure stays with me¡ I mentioned that because Martin suggested to use a small transmitting loop antenna at some distance from a constructed receive antenna measuring with VNA S21 parameter of combined antenna and preamplifier arrangement. In my days I used a good calibrated log-periodic transmit antenna for similar measurements to assess quality of receiving antennae. ? Raphael ? From: [email protected] <[email protected]> On Behalf Of W0LEV
Sent: Monday, December 5, 2022 4:49 PM To: [email protected] Subject: Re: [loopantennas] Measuring S11 or frequency range of an active SRL ? Thanks, Raphael!? In the EMC world in which I worked and still consult, we pretty much universally use the 2D^2 / Lamda.? Yes, for antennas which are small as a function of wavelength, wavelength / 2pi is a far better approximation of fr field. ? Dave - W?LEV? ? On Mon, Dec 5, 2022 at 9:39 PM Raphael Wasserman <wassermanr46@...> wrote:
Dave - W?LEV ? ? |
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Re: Measuring S11 or frequency range of an active SRL
Thanks, Raphael!? In the EMC world in which I worked and still consult, we pretty much universally use the 2D^2 / Lamda.? Yes, for antennas which are small as a function of wavelength, wavelength / 2pi is a far better approximation of fr field. Dave - W?LEV? On Mon, Dec 5, 2022 at 9:39 PM Raphael Wasserman <wassermanr46@...> wrote: Dave, -- Dave - W?LEV |
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Re: Measuring S11 or frequency range of an active SRL
Dave,
Your feedline can be proprely terminated at preamp input but the input of the same feedline is not properly matched to the impedance of your antenna. Of course, when you deal with a reasonable high antenna noise you may turn off light and enjoy a book reading instead... Regarding your second observation, the mentioned by you far field distance is correct for electrically large antennae but for electricaly small antenae the far end region can be esimated as d > wavelength / 2 x pi. Regards, Raphael |
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Re: Measuring S11 or frequency range of an active SRL
Another easier and more empirical approach: If you sense reasonable antenna noise (=/> 6 dB) when the antenna feedline is connected to the preamp over a properly terminated preamp input (the value on the complex impedcance of the loop as a function of frequency), why bother with matching?? Under those conditions, I believe establishing a good "match" is irrevalent. Further and addressing the method above and considering the transmsit antenna must be in the far field of the receive antenna, generally we use 2D^2 / [free space wavelength].? D is the largest dimension of the transmit antenna.? This is pretty general, but can be arrived at with some complex math. Dave - W?LEV? ? On Mon, Dec 5, 2022 at 7:08 PM Raphael Wasserman <wassermanr46@...> wrote: Jinze, -- Dave - W?LEV |
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Re: Measuring S11 or frequency range of an active SRL
Error be corrrected in equation
?Re [ S11 ] = ( R ^2 -50 ^2 + X ^2 )/ [ ( R + 50 ) ^ 2 + X ^ 2 ]? |
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Re: Measuring S11 or frequency range of an active SRL
Jinze,
Unfortunately, your response is giberish because it can be made two independent measurements of S11 parameter regarding the frequency range of interest and related either to the antenna under the test or S11 parameter related to the input of antenna amplifier using VNA. Yes, since VNA's port 0 has constant 50 ohm resisistive impedance value, we measure how good the input impedance of amplifier matches this 50 ohm value or the antenna under the test matching the same 50 ohm value. In this test we are not assessing true loop antenna reception but weighing whether it pays to match antenna impedance to the input of selected amplifier.? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? However, using VNA with a transmit antenna connected to port 0 and receive antenna to port 1 you can measure S21 scattering parameter, observing balance to unbalance conditions between your antennae and VNA unbalance ports. There is a possibility even indirectly calculate Gain of? antenna in the frequency range of interest. Only one difficulty is to? observe? - the measurements shall be done in the far field of your transmit antenna ( for electrically small antenna ): d > wavelength of interest/ 2 x pi, where d - distance in meters The calculated then?G antenna = 4 x pi x d x sqrt | S 21 | / wavelength of interest. Also, there is known relation between [S11] scattering parameter parts Re [S11] + Im [S11] and? impedance Z value = R + j X . You can convert/transform? them like Re [S11] = ( R ^ 2 - 50 ^ 2 + X ^ 2 ) / [ ( R + 50 0 ^ 2 + X ^ 2 ]? and Im [ S11] = 2 x 50 x X / [ ( R + 50 ) ^ 2 + X ^ 2 ]. Raphael |
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Re: Measuring S11 or frequency range of an active SRL
?
Yes, due to the VNA's 50 ohm constant impedance, you won't be able to get true loop reception, but at least you can see a gain level of the amplifier, and you can scan out the input impedance of the amplifier. I think this is whether the amplifier is suitable for The key to the loop |
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Re: Measuring S11 or frequency range of an active SRL
On Mon, Dec 5, 2022 at 08:48 AM, jinze peng wrote:
I try to use a 1:1 voltage balun as the input coupler of the small loop amplifier, so that the gain sweep can be performed directly through the NANOVNA, and I will connect a -10db attenuator in series with each of the two ports of the nanovna to prevent signal overloadUnfortunately this doesn't present the same source impedance to the amplifier as that of an actual loop. You will obtain a fairly flat amplitude response, but this is just the amplifier and is not indicative of the actual system performance. Regards, Martin |
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Re: Measuring S11 or frequency range of an active SRL
I try to use a 1:1 voltage balun as the input coupler of the small loop amplifier, so that the gain sweep can be performed directly through the NANOVNA, and I will connect a -10db attenuator in series with each of the two ports of the nanovna to prevent signal overload
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Re: Measuring S11 or frequency range of an active SRL
Dave and Dirk,
Loop/ antenna amplifier for small magnetic loop antennas ( for any small antenna ) does not serve as an impedance compensation network unless you use a non - Foster method of impedance compensation or parametric amplifier. Ordinary loop amplifier compensates for low effeciency of antenna due to its low radiation resistance. Dirk constructed a loop antenna and antenna amplifier. In order to ascertain the amplitude frequency response of combined antenna and amplifier Martin suggested to run S21 measurement using VNA. If this test shows unsatisfactory response in frequency band of his interest, Dirk should use S11 parameter set of his designed antenna that can be utilized for a design and simulation of matching impedance network between his antenna and NE562 antenna amplifier. For example, Chavdar used a simplistic impedance matching network ( shown as a plot of combined S21 with filter in Martin's recent reply ).? Raphael ? ? |
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Re: Measuring S11 or frequency range of an active SRL
¿ªÔÆÌåÓýThe best way of testing an active loop is in a Crawford Cell but
only a few National Labs have big enough cells to take a 1m loop ,
it also costs a bit to hire. Using a small TX loop close to the RX loop is perfectly valid
provided that the TX loop current is uniform, you do that by
keeping the loop small and using a series resistor to swamp the
small loop inductance.? The old NBS method of measuring
sensitivity used a current meter to measure the loop current but
the swamp resistor with a constant voltage drive works good
enough. People that do much loop testing might just build a test emitter,
this can be a 1MHz xtal osc, a divider and monostable to produce a
short ( <5nS or so) pulse into a small loop so you get a
constant amplitude comb. You hang the gizmo in the centre of the
loop then a spectrum analyser ( or a SDR ) displays the response,
handy for quick verification. But whatever you use to inject the
signal it needs to be small enough not to overload the amplifier
otherwise you can get totally misleading results. But you can
drive a small TX loop from a Tracking Generator. I have also -at
times- used a wire loop in my Lab to send a signal down to the
bottom of the garden loop to check that the active loop, feeder,
BPF and distribution amps are working. People used to make larger-than -the- loop-being -tested
Helmholtz coils to produce a very uniform field, that's good for
ferrite rod antennas too however a single TX loop at a set
distance is "Industry Standard" for receivers with internal
ferrite or wire loop antenna sensitivity measurements. There is an
inverse cube factor for the loop to rod distance, but the
Helmholtz uses two loops so as you move away from one you move
closer to the other. However the loop is a very simple circuit element well below
resonance so there really isn't much point in testing the loop and
amplifier unless you are doing advanced stuff,? testing the
amplifier with a dummy loop works fine providing that the dummy
represents the loop sufficiently well. Indeed there are very good
reasons for NOT testing on-air, I get around 800mV/m in MW and the
usual urban noise so testing on a dummy in a screened box is the
only way to reach system noise levels for an experimental
amplifier and to perform IMD measurements. Our Bulgarian Friend? LZ1AQ did make a very low power signal
source using a short whip antenna, that was a casual "check it's
working test", not a measurement. I don't think there is any point
in setting up a distant transmitter unless you use a reference
antenna close to the loop under test and you do a comparison
between the two receive antennas, an E-Field antenna could be a
good choice. But you don't need far field conditions at all, just
an equal field at both antennas. Incidentally the comparison of
loop heights above ground mentioned earlier sent me looking at lab
notes made 46 years ago when I did almost the same test with very
similar results. Since then I usually run small loops just above
soil level for VLF to HF, I also wrap black duct tape ( or Camera
Tape ) around? loops ( or use black wire) , you get far fewer
complaints from Higher Management when the shine is covered up. I was looking at the latest batch of CN cheap loops earlier
today, there was a video amp IC on a PCB with a gain pot, bit like
a previous effort but this one left out the switcher so it might
be a little better than pathetic. Regards Alan G8LCO
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Re: Measuring S11 or frequency range of an active SRL
Since s11 is a reflective measurement (think return loss) and from the plot, it appears the loop amplifier is more of an impedance compensation/stabilization? network. Dave - W?LEV On Sun, Dec 4, 2022 at 9:00 AM Martin via <martin_ehrenfried=[email protected]> wrote: Hi Dirk, -- Dave - W?LEV |
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Re: Measuring S11 or frequency range of an active SRL
Hi Dirk
I don¡¯t hink Chavdar has done that, atleast not on separate frequencies. I may be wrong. He has set up a transmitter in far field on 160m but that was for testing the delay line phased loops. You can find that on his website under delay line or phased loops.( not sure which) but was for testing b/f ratio. Simon |
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Re: Measuring S11 or frequency range of an active SRL
Thank you Mike!
I found an article about measuring the difference of the mounting height of SMLs using existing signals from 77 kHz to 28 MHz here: Would you pls. point me to the LZ1AQ setup with test transmitter in the far field? Could I perhaps also use my NanoVNA for this purpose (then S21) by connecting the SML to one (receiving) port and a small coil to the other VNA port (transmitting)? |
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Re: Measuring S11 or frequency range of an active SRL
Hi Dirk,
They way I have done it in the past whilst trying to optimise loop and loop amplifier setups, was to place a small loop (as small as possible) within the main loop, and feed the small loop via a 3dB attenuator (to keep the signal source happy) and then sweep the overall loop response. Although this is not the same as the far field response, it does seem to produce a very close approximation to the actual performance. This is the sort of plot you can obtain. Regards, Martin |
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Re: Measuring S11 or frequency range of an active SRL
The proper way would be to set up a calibrated test transmitter in the far field zone and sweep its frequency. You can read how LZ1AQ did it on his website.
The gain (or more correctly, antenna factor) variation across the wide range will be way more than 3dB, more like 30dB. But it is still an interesting experiment. 73, Mike AF7KR |
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Measuring S11 or frequency range of an active SRL
Perhaps a silly question:
What is the best way to measure the frequency range of an active SRL? I have built a 1m diameter loop with 10mm^2 copper with a NE592 amplifier. First tests show, that it works good from VLF to middle VHF. But I would like to measure the frequency range (-3dB). I have no very special equipment. What I could use: a NanoVNA, a signal generator from 10 kHz to 30 MHz (no sweeping function), some SDR receivers ... |
Dirk